Effect of elevated CO2 concentration and nitrate: Ammonium ratios on gas exchange and growth of cassava (Manihot esculenta Crantz)

Abstract

Aims: This study evaluated how different nitrogen forms affect growth and photosynthetic responses of cassava to CO2 concentration. Methods: Cassava was grown in 14-L pots in a greenhouse at 390 or 750 ppm of CO2. Three nitrogen treatments were applied: (a) 12 mM NO3-, (b) 6 mM NO3- + 6 mM NH4+, and (c) 12 mM NH4+. Results: Thirty-six days after treatments began, plants grown under elevated CO2 and fertilized only with NO3- (750_NO3-) had photosynthetic rates similar to plants grown under 390_NO3-, indicating significant photosynthetic acclimation to CO2. In contrast, photosynthetic rates at elevated CO2 increased as NH4+ increased in the nutrient solution, such that photosynthetic acclimation was reduced for plants fertilized with only NH4+. However, this positive effect of NH4+ on photosynthesis was not observed in more advanced growth stages, and the toxic effects of NH4+ severely reduced total dry mass for these plants measured at the end of the experiment. Conclusions: Our results indicate that cassava will respond with increased biomass accumulation in response to raising atmospheric CO2 levels, and that N form can have an important impact on the photosynthetic response. However, the positive effect of NH4+ fertilization on cassava photosynthetic CO2 response eventually led to a toxicity problem that reduced biomass production. The challenge is to determine how to manage NH4+ fertilization so that the photosynthetic benefit observed in the initial phase may persist throughout the crop cycle. © 2013 Springer Science+Business Media Dordrecht.